Catalysts used for purification of the exhaust gas emitted from automobiles, etc. must be heated to a desired temperature or higher by, for example, the heat of exhaust gas, in order to exhibit their catalytic activities. Therefore, when the temperature of exhaust gas is low as experienced during the cold start of engine, the harmful substances (HC, CO and NOx) in exhaust gas are hardly purified. HC, in particular, is discharged in a large amount during the cold start, and its purification is an important task to be achieved.
To improve the efficiency of HC purification during the cold start, a technique is known which comprises using, as a HC adsorbent, a molecular sieve made of a crystalline aluminosilicate (e.g. a zeolite) and allowing the adsorbent to adsorb and store HC until a catalyst reaches its operating temperature.
For example, an apparatus for purification of automobile exhaust gas, using Y zeolite or mordenite as a HC adsorbent is disclosed in Japanese Patent Application Kokai (Laid-Open) No. 75327/1990. Also in Japanese Patent Application Kokai (Laid-Open) No. 293519/1992 is asserted use of an adsorbent obtained by subjecting H.sup.+ /ZSM-5 zeolite to ion exchange with Cu and Pd, in order to alleviate the adverse effect by water adsorption and achieve higher HC adsorption capability and wider temperature range allowing for HC adsorption. For the same purpose, use, as an adsorbent, of a pentasil type metalosilicate subjected to ion exchange with H, Cu or Pd is proposed in Japanese Patent Application Kokai (Laid-Open) No. 63392/1994.
Zeolites, etc. heretofore used as a HC adsorbent, however, are inferior in heat resistance, particularly in heat resistance in a moisture-containing atmosphere (e.g. an automobile exhaust gas), i.e. hydrothermal resistance, as compared with alumina, etc. used as a carrier of ordinary catalyst for automobile exhaust gas purification; therefore, there has been a fear that they deteriorate when used at high exhaust gas temperatures such as experienced in continuous high speed driving of automobile.
Use of zeolite particularly in an in-line type exhaust gas purification system (which uses no bypass for high temperature exhaust gas and has a simple structure) has been fairly severe to ordinary zeolite.
The present invention has been completed in view of the above-mentioned problem of the prior art. The object of the present invention is to provide an adsorbent suitably usable in applications where the adsorbent must have high heat resistance and high hydrothermal resistance, for example, a HC adsorbent used in a system for hydrocarbon adsorption or for purification of exhaust gas from internal combustion engine, such as an in-line type system for exhaust gas purification.